Abstract Recent clinical evidence has demonstrated that immunotherapy is a powerful therapeutic strategy for cancer. However, the immune suppressive tumor microenvironment (TME) posts immense challenges for such approach. Among many immune tolerance mechanisms associated with tumor progression, lack of appropriate ?danger? signal in TME is chiefly responsible for failure to trigger and sustain tumor immune surveillance. Interleukin-33 (IL-33) is a member of the IL-1 family and is released by injured tissue cells or activated innate immune cells such as macrophages upon tissue damage or infection. Thus, IL-33 is thought to serve as an endogenous ?danger signal?, which triggers inflammation and cell-mediated immune responses in affected tissues. We have found that IL-33 is expressed in many normal human lining tissue cells but drastically down- regulated in advanced cancer cells. These data are consistent with a role of IL-33 in inhibiting tumor progression. Our study has further revealed that ST2, a receptor for IL-33, is highly expressed in CD8+ tumor infiltrating lymphocytes (TIL) as well as in effector CD8+ T cells in a T-bet dependent manner. In addition, we demonstrated that expression of IL-33 in tumor cells strongly inhibited tumor growth in mice without affecting tumor cell proliferation in vitro and that the antitumor effect of IL-33 is dependent on CD8+ T cells and NK cells. We hypothesize that inflaming tumor with the immunological ?danger signal? IL-33 has strong antitumor effect and can serve as a new cancer immunotherapy. Following aims are proposed to test this hypothesis: Specific Aim 1. We will test the hypothesis that arming oncolytic virus with IL-33 promotes antitumor immunity. Specific Aim 2. Determine whether the immune ?checkpoint? immunotherapy synergizes with tumoral expression of IL- 33 to further enhance antitumor immunity. The proposed studies are originated from our recent novel findings, highly clinically relevant, and should pave way for clinical translation of this new antitumor cytokine.